Technology computer-aided design (TCAD)-based virtual fabrication

ABSTRACT

A single finite element mesh is generated for predicting performance of an integrated circuit design. A plurality of sample points are identified for conducting a variability study on at least one parameter associated with the integrated circuit design. The sample points are selected to predict performance of the integrated circuit design when subject to variations in the at least one parameter due to variations in manufacturing processes to be used to manufacture the integrated circuit design. A parameterized netlist is generated corresponding to each of the sample points. A technology computer aided design (TCAD) simulation is run for each of the parameterized netlists, using the single finite element mesh for each of the parameterized netlists, until convergence is achieved, to obtain, for each of the parameterized netlists, at least one metric indicative of the performance of the integrated circuit design. A predicted design yield is developed for the integrated circuit design.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/820,741, filed Jun. 22, 2010, incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to the electrical, electronic and computerarts, and, more particularly, to computer-aided design of electroniccircuitry and the like.

BACKGROUND OF THE INVENTION

Chip design with iterative fabrication cycles is expensive and quiteslow. Fabrication steps involve processes such as lithography, doping,etching, chemical mechanical polishing, and the like. Manufacturingprocesses, however, are not deterministic, and have inherent random andsystematic variability built into them. Design-for-Manufacturing (DfM)methods can predict and/or project fabrication results usingfully-software-based tools. Technology Computer-Aided Design (TCAD)tools can be one of the key components of DfM. However, numerical devicesimulations (i.e., TCAD) are based on solving drift-diffusion orhydrodynamic transport equations which are forms of partial differentialequations. One example of such simulations is the MEDICI: 2-D DeviceSimulation tool available from Synopsys Inc., Mountain View, Calif.,USA.

Furthermore, device and/or circuit mix-mode simulations consumeadditional effort and more time, so TCAD tools cannot be directly usedin circuit analysis. TCAD-embedded statistical analysis cannot beapplied in the DfM world due to computational time and a weak TfM (TCADfor manufacturability) infrastructure. TCAD simulations and methods haveexpensive computational times and cannot be directly inserted into DfMflows.

Prior art techniques have focused on “SPICE compact models” for thedevice or the device variability to study design yield.

SUMMARY OF THE INVENTION

Principles of the invention provide technology computer-aided design(TCAD)-based virtual fabrication. In one aspect, an exemplary methodincludes the steps of generating a single finite element mesh forpredicting performance of an integrated circuit design; and identifyinga plurality of sample points for conducting a variability study on atleast one parameter associated with the integrated circuit design. Thesample points are selected to predict performance of the integratedcircuit design when subject to variations in the at least one parameterdue to variations in manufacturing processes to be used to manufacturethe integrated circuit design. Additional steps include generating aparameterized netlist corresponding to each of the sample points; andrunning a technology computer aided design simulation for each of theparameterized netlists, using the single finite element mesh for each ofthe parameterized netlists, until convergence is achieved, to obtain,for each of the parameterized netlists, at least one metric indicativeof the performance of the integrated circuit design. A further stepincludes developing a predicted design yield for the integrated circuitdesign, based on the at least one metric determined for each of theparameterized netlists.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

One or more embodiments of the invention or elements thereof can beimplemented in the form of a computer product including a computerreadable storage medium with computer usable program code for performingthe method steps indicated. Furthermore, one or more embodiments of theinvention or elements thereof can be implemented in the form of anapparatus including a memory and at least one processor that is coupledto the memory and operative to perform exemplary method steps. Yetfurther, in another aspect, one or more embodiments of the invention orelements thereof can be implemented in the form of means for carryingout one or more of the method steps described herein; the means caninclude (i) hardware module(s), (ii) software module(s) executing on oneor more hardware processors, or (iii) a combination of hardware andsoftware modules; any of (i)-(iii) implement the specific techniques setforth herein, and the software modules are stored in a computer readablestorage medium (or multiple such media).

One or more embodiments of the invention may offer one or more technicalbenefits, such as a fast and efficient TCAD methodology with aninterface to a fast statistical method, with no need to rely onempirical inputs.

These and other features, aspects and advantages of the invention willbecome apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating exemplary techniques for TCAD formanufacturing of a process-sensitive circuit, according to an aspect ofthe invention;

FIG. 2 shows an exemplary interface between TCAD and a statisticalsampling technique, according to another aspect of the invention;

FIG. 3 shows an exemplary dynamic client-server implementation of TCADand a fast sampling technique, according to still another aspect of theinvention;

FIGS. 4 and 5 present exemplary TCAD simulations;

FIG. 6 depicts a computer system that may be useful in implementing oneor more aspects and/or elements of the invention;

FIG. 7 shows another exemplary interface between TCAD and a statisticalsampling technique (importance sampling), according to still anotheraspect of the invention; and

FIG. 8 illustrates an aspect of the invention wherein meshes requiresolving partial differential equations for the finite element method butthe TCAD physical model does not need additional repeated meshgeneration processes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As noted, chip design with iterative fabrication cycles is expensive andquite slow. Fabrication steps involve processes such as lithography,doping, etching, chemical mechanical polishing, and the like.Manufacturing processes, however, are not deterministic, and haveinherent random and systematic variability built into them.Design-for-Manufacturing (DfM) methods can predict and/or projectfabrication results using fully-software-based tools. TechnologyComputer-Aided Design (TCAD) tools can be one of the key components ofDfM. However, numerical device simulations (i.e., TCAD) are based onsolving drift-diffusion or hydrodynamic transport equations which areforms of partial differential equations. One example of such simulationsis the MEDICI: 2-D Device Simulation tool available from Synopsys Inc.,Mountain View, Calif., USA.

Furthermore, device and/or circuit mix-mode simulations consumeadditional effort and more time, so TCAD tools cannot be directly usedin circuit analysis. TCAD-embedded statistical analysis cannot beapplied in the DfM world due to computational time and a weak TfM (TCADfor manufacturability) infrastructure. TCAD simulations and methods haveexpensive computational times and cannot be directly inserted into DfMflows.

One or more embodiments advantageously provide fast and accurate circuitsimulations and analyses which permit capturing rapidly-changed devicefeatures and intrinsic device fluctuation, as well as random mismatchamong adjacent devices. One or more embodiments provide techniques forimproving circuit design and production for manufacturing yield. In oneaspect of the invention, a method for TfM of a process-sensitive circuitis provided. Based on physical and process variability sources, avirtual representation of a fabricated system is obtained using physicalTCAD parameters that can account for process variations during a circuitproduction process. The physical TCAD parameters are modified to helpimprove the circuit and/or the production process. These steps arerepeated until a desired projected production yield is attained.

One or more embodiments of the invention provide a robust and fast TfMframework by interfacing TCAD and fast importance sampling (faststatistical) techniques and to convert device variability sources toTCAD parameters for efficient and fast statistical analysis, which is aninnovative TCAD simulation method. A non-limiting example of fastimportance sampling (fast statistical) techniques is set forth in:

-   -   R. Kanj, R. Joshi, & S. Nassif, “Mixture importance sampling and        its application to the analysis of SRAM designs in the presence        of rare failure events,” Design automation conference, 2006.    -   U.S. Pat. No. 7,380,225 of Joshi et al. entitled “Method and        computer program for efficient cell failure rate estimation in        cell arrays.”

The skilled artisan will be familiar with such techniques; nevertheless,the complete Kanj et al. paper and the complete disclosure of U.S. Pat.No. 7,380,225 are expressly incorporated herein by reference in theirentirety for all purposes. The skilled artisan will of course realizethat an SRAM is a static random access memory.

Advantageously, one or more embodiments, as compared to past techniques,are much more accurate and efficient to characterize and predictcircuits and yields, and they are applicable to variety of variationssuch as, by way of example and not limitation, random-dopant variation,line-edge roughness, oxide fluctuations, time-dependent reliability,supply voltage (V_(dd)), cell-supply voltage (V_(cs)), L, W, andtemperature (L and W, gate length and channel width respectively, arenon-limiting examples of device dimensions).

FIG. 1 is a diagram illustrating a methodology for TfM of aprocess-sensitive circuit, in accordance with an aspect of theinvention. The term “process-sensitive circuit,” as used herein,generally includes any circuit whose function and/or quality can beaffected by the circuit production process. Exemplary process-sensitivecircuits include, but are not limited to, memory cell arrays, such asstatic random access memory (SRAM) and dynamic random access memory(DRAM) cell arrays. As will be described in detail below, theperformance of memory cells, such as SRAM, can be affected by variationsthat can occur during a circuit production process, such as randomdopant fluctuations (RDFs) which can result in threshold voltage (V_(T))variations and patterning process variations, e.g., lithographyvariations. As shown at 102, variability sources can include (again byway of example and not limitation) gate length (L), channel width (W),RDF, LER (line edge roughness), oxide thickness (T_(ox)) parasiticand/or distributive capacitance and/or resistance, supply voltage(V_(dd)), cell-supply voltage (V_(cs)), chip temperature, designparameters, and aging effects (negative bias temperature instability(NBTI), positive bias temperature instability (PBTI), and hot carriersinjection (HCI)).

As shown at 104, a TCAD device structure can be generated in lieu ofmany mesh structures depending on the variability sources. Then, TCADinput parameters 106 can be physically evaluated based on modelequations and device structures. As shown at 108, the parameters can bedirectly used in importance sampling, such as MIS (mixture importancesampling) statistical calculations, which can be applied, for example,to analysis of memory cell and/or circuit performance, power, and/orstability, as at 110; yield, as at 112; and design quality metric(s), asat 114. Given the teachings herein, the skilled artisan will be able toadapt techniques from the aforementioned U.S. Pat. No. 7,380,225 and/orthe aforementioned Kanj et al. paper to implement importance sampling inone or more embodiments of the invention.

One or more embodiments of the combined interface 108 may provide one ormore of the following advantages:

-   -   The fast statistical analysis enables estimating low fail        probability with a reasonably low number of samples involved.        This makes it more practical to perform statistical analysis at        the mixed-mode level, and therefore allows for the usage of        TCAD-based statistical design and yield analysis.    -   The TCAD mixed mode analysis enables more physical (i.e., based        on underlying physics by solving (typically in discretized form)        underlying partial differential equations, as opposed to using        empirical correlations) and accurate analysis of the effects of        statistical process parameter variation on the yield. Such        effects may not be well abstracted or captured with traditional        process models or SPICE-like circuit simulation-based        approaches. This aspect includes but is not limited to the:        -   ability to properly model nonrectangular device shapes;            existing efforts to model based on table-based abstractions            are designed to match only certain regions of the device            characteristics.        -   complex interaction of different process parameters whose            effects may not be well modeled in existing device models            like stress, mobility mismatch and other device            characteristics.        -   Furthermore, with the increase in variability, the process            parameter variations often stretch beyond the modeling            limits, thereby in a traditional approach, designers often            rely on extrapolations of the models to account for such            large variations; a TfM approach in accordance with one or            more embodiments of the invention captures the true and            physical (again, based on underlying physics by solving            (typically in discretized form, such as via the finite            element method) underlying partial differential equations,            as opposed to using empirical correlations) device            characteristics.    -   This approach can enable on the fly process and/or memory design        yield optimizations even if real hardware is not available. For        example, it is possible to study the stability of the SRAM cell        if it is decided to change some halo implants, special types of        doping, and so on.    -   Finally, depending on the available abstractions and the desired        process variations to be studied, the approach can be used in a        hybrid fashion, where mixed-mode analysis can call some        simplified models, or call TCAD based on the criticality of        and/or availability of models for the type of process variation        being modeled. This approach is device characteristics based.    -   Alternatively, it is possible to construct a fail-region based        analysis, where critical samples that fall on the boundary are        analyzed by mixed-mode techniques, whereas samples that strongly        pass or strongly fail can be analyzed with SPICE-like methods.        By way of further explanation, during statistical modeling,        consider a rare failure mode region (larger σ). In one or more        instances, initially, sample uniformly from, e.g., −6σ to +6σ.        Assign uniform probability to each transistor and determine        probability of failure. Within a cluster, locate the center of        gravity and shift the Gaussian distribution and sample in the        region of a strong fail. See FIG. 7 and accompanying discussion        below.    -   In an alternative embodiment, compare track over the region of        variability TCAD/SPICE-like regions of agreement and use that to        guide TCAD simulation requests. That is to say, because SPICE is        relatively fast and TCAD is relatively slow, seek to determine        important regions where TCAD is to be used and use SPICE        elsewhere.

FIG. 2 shows the proposed TCAD-statistical sampling interface. As shownat 202, build a fixed mesh for device simulation. In one or moreembodiments, rather than changing the mesh each time for changes in aparameter (e.g., dopant placement or any of a number of other parameterssuch as those discussed above), use a single mesh for all runs andchange parameter values such as dopant density, mobility, or the like.In step 204, identify a set of statistical samples using a suitablestatistical method, such as Monte Carlo, sensitivity analysis,importance sampling, or the like. By way of example and not limitation,initially sample the parameter space uniformly. Run TCAD for each one ofthe samples, as at step 206, on a netlist modified for the specificparameter values. Such a parameterized TCAD netlist will include circuitelements, their corresponding nodes, and one or more parameters, e.g.,dopant density, work function, and the like, and their correspondingvalues. One non-limiting example of a TCAD input file includes p_in vals(parameter values) in the TAURUS MEDICI software available fromSynopsys, Inc., Mountain View, Calif., USA (SENTAURUS software fromSynopsys is another non-limiting example of suitable software). Again,instead of changing the mesh, in one or more embodiments, change theparameters. Each simulation (i.e., for each statistical sample) is basedon its own parameterized netlist, and each of the parameterized netlistschanges the values of suitable parameter(s) such as dopant density, workfunction, or the like, rather than changing the mesh. In each simulationfor each different parameterized netlist corresponding to each differentstatistical sample, run TCAD on the given parameterized netlist, as atstep 206. The result is then the transient (or AC) waveforms of thestudied samples; note that the type of study (DC and/or AC) is based onthe designer's intent. One or more metrics of interest are evaluatedfrom the waveform(s) for each sample. An example of a metric is themaximum noise on a given node. A determination is made in step 208 as towhether convergence has occurred (i.e., within a particularparameterized netlist, run until convergence; once converged, check andsee if there are still more samples to be run). For example, if using astandard Monte Carlo technique, continue to simulate until the estimatereaches convergence. In the case of importance sampling, as in FIG. 7,to be discussed further below, first undertake uniform sampling on oneor several thousand simulations, then obtain a center of gravity offails and undertake shifted Monte Carlo sampling to carry out importancesampling in a second jump and check for convergence.

As shown at 210, the result is a TCAD-aware design yield.

For memory designs, yield analysis has often focused on dynamic(transient) behavior of the design. The results for statisticalsimulations are then used to identify fails based on predefined usercriteria which then are used to compute probabilities. The importancesampling phase includes multiple statistical analysis stages; typically,one that searches for critical fails and others that focus on the failregions. Thus for each stage, the process is continued until enoughsamples are studied to satisfy certainty.

FIG. 7 shows a flow chart analogous to that of FIG. 2 but forimportance-based sampling. Step 702 is identical to step 202. In step704, develop the samples for a first loop (typically uniform sampling).Step 706 is analogous to step 206. In step 708, compute the center ofgravity for the fails predicted from step 706. In step 710, build newsamples for a second loop, using shifted importance sampling based onthe identified centers of gravity for fails. Step 712 is analogous tostep 706 but for the revised samples. It is repeated until convergenceis achieved, as per step 714. The result is yield estimate 716.

As shown with respect to FIG. 3, methods can be performed in parallelper loop with client-server protocol embedded with dynamic loadassignment based on server availability. That is to say, for multiplesamples, the job can be parallelized. FIG. 3 depicts a dynamicparallelization (dynamic load assignment strategy). Suppose, forexample, there are ten machines 302, numbered 1 to n. Each machine canhave code that will wait for the next sample. The master machine 304running the main job will look over the pool of samples and determinewhich machine 302 is available for the next simulation; that machinewill be provided with a new “p_in vals” to simulate and a correspondingnew p_out will be obtained. Because the runs for different values of theparameters (different samples) do not depend on each other, the problemis readily parallelized. That is to say, if runs were to be conductedfor dopant densities of, say, Density₁, Density₂, and Density₃, the runfor Density₂ does not depend on that for Density₁ or Density₃. Theoverall process is depicted at the upper right-hand corner of FIG. 3,wherein new p_in vals are provided at 306, simulation is conducted forthose values at 308, and the corresponding outputs p_out are analyzed at310.

One or more embodiments of the invention thus provide a TCAD-importancesampling interface tool to optimize circuits (such as SRAM circuits) andmanufacturing yield. Further, one or more embodiments provide TCADinnovation by varying physical parameters without regenerating meshprocess for variability study. In at least some instances, fastnumerical Monte Carlo simulations can be embedded in TCAD-based DfMmethods. Furthermore, one or more embodiments provide technology and/orprocess-compatible TCAD simulation and analysis, and/or analysis methodsof process parameters for DfM impact.

One or more instances of the invention provide a robust and fast TfMframework by interfacing TCAD and fast importance sampling (faststa(istical) techniques (e.g., MIS); and/or convert device variabilitysources to TCAD parameters for efficient and fast statistical analysisin an innovative TCAD simulation method.

One or more embodiments do not rely on empirical inputs, and thus areable to avoid the very long step and inaccurate results encountered withTCAD-to-SPICE techniques (due to the empirical SPICE parameters employedin such techniques). One or more embodiments advantageously provide theability to perform immediate device and/or circuit simulations topredict yield, unlike techniques having long and complex simulationcycles.

It should be noted that, unlike one or more current techniques which useempirical approximations for the L and W parameters, one or moreembodiments use the true L and W values. Advantageously, one or moreembodiments thus avoid the issues associated with L and Wapproximations, which are typically inadequate for use with memories andanalog or RF (radio frequency) circuitry. SRAM-like circuits operate inan analog fashion, in that the devices may be in the linear orsaturation regions, such that matching only the off and on currents isinsufficiently accurate. Further, one or more embodiments do not need toemploy error prone compact models. In one or more embodiments, thelithographic contours can be fed to the TCAD tool, and mix-modesimulation can address the layout-dependent variability. Furthermore,one or more embodiments advantageously provide flexibility in thepresence of novel device recipes.

It is believed that one or more embodiments can benefit fromcomputational parallelization, as per FIG. 3.

Thus, one or more embodiments provide speed-up of TCAD for in situ faststatistical TCAD analysis of the circuit understudy and/or faststatistical TCAD analysis for the yield of circuit designs. One or moreembodiments include a fast statistical method with physical TCADparameters-based methods. One or more embodiments do not require devicemodel abstractions. One or more embodiments do not require otherthree-dimensional (3-D) simulators for statistical variability analysis.

Note that the “speed-up of TCAD” embedding statistical technique isquite significant in one or more embodiments.

Again, one or more embodiments provide fast and accurate methods of TCADwith interfaced statistical methods.

Examples

A non-limiting exemplary simulation was carried out using an embodimentof the TCAD-to-importance sampling interface. Due to the reduced TCADsimulation cycle and fast importance sampling, the exemplary simulationreduced the number of simulations from millions to a few thousand. FIG.4 shows TCAD device and/or circuit mix mode simulations for a simplifiedembedded dynamic random access memory (eDRAM) structure 402. Structure402 includes word line 404, bit line 406, access transistor 408, cellnode 410, and trench capacitor 412 between node 410 and ground. Thesimulation was conducted for a Partially Depleted Silicon-On-Insulator(PDSOI) DRAM with a trench capacitance of 20 femtofarads (fF) and a bitline capacitance of 200 fF. The word line, bit line, and simulated cellnode voltages are shown as a function of time in seconds. Theabove-described TCAD-importance sampling interface is capable ofanalyzing variations in many different parameters; purely for the sakeof illustration and not limitation, trench capacitance was analyzed as aparameter of interest in this example.

A non-limiting exemplary yield test simulation was also carried outusing an embodiment of the TCAD-to-importance sampling interface. TheeDRAM cell 402 was used as an example. As shown in FIG. 5, for the lowerstorage (trench) capacitance (around 5 fF), the data storing operationin the eDRAM can fail. Curve 551 is for 20 fF, curve 553 is for 10 fF,and curve 555 is for 5 fF. Curve 555 is too close to logical zero(whereas, ideally, it should be at or close to a logical one) and thus afailure can ensue. In this example, the above-mentioned metric (e.g.,steps 206, 706, 712) is whether the eDRAM is capable of having a logicalone written to it or not. The curves for the cell node in FIG. 5 areexamples of the above-mentioned waveforms (e.g., steps 206, 706, 712).While not depicted in the figure, exemplary experimental simulationshave demonstrated that TCAD-aided methods can also predict the failureof eDRAMs due to lithographical and electrical variations.

In one or more embodiments, the simulation time is on the order ofhours, whereas current workflows may involve weeks of simulations. Forexample, a mixed-mode simulation with simplified TCAD parameterdistributions from hardware V_(T) distribution and dopant densitydistribution takes about 2-3 minutes per simulation. Importance sampling(Fast Monte Carlo) reduces the number of simulations from millions to afew thousand. The experimental eDRAM example mentioned above ran on twoclient/server connections in 14 hours, with assumed work functionvariability and assumed trench capacitance variability. Currenttechniques with TCAD device simulation plus mixed mode simulation for avery basic circuit, such as a one-stage inverter, with a reasonableamount of mesh, can take on the order of a day; furthermore, currentMonte Carlo techniques require millions of samples. It is to beemphasized that the exemplary run times are presented for purelyillustrative purposes and different results may be obtained with otherembodiments of the invention and/or when simulating different problems.

It is worthwhile observing that in current techniques, exemplary inputsto a SPICE (empirical equations not modeling the underlying physics withthe finite element method) analysis could include mobility (μ₀), andempirical (i.e., not actual) values for L and W, as well as compactmodels. The empirical values for the device dimensions may vary for offand on currents, due to variations inline edge roughness as technologyscales. However, in one or more examples employing embodiments of theinvention, the actual device characteristics (e.g., mobility (μ₀)) andactual device dimensions are employed, modeling the underlying physicswith the finite element method, so the aforementioned errors are notintroduced as technology scales.

Recapitulation

Given the discussion thus far, it will be appreciated that, in generalterms, an exemplary method, according to an aspect of the invention,includes the step 202, 702 of generating a single finite element meshfor predicting performance of an integrated circuit design. A furtherstep 204, 704 includes identifying a plurality of sample points forconducting a variability study on at least one parameter associated withthe integrated circuit design. The sample points are selected to predictperformance of the integrated circuit design when subject to variationsin the at least one parameter due to variations in manufacturingprocesses to be used to manufacture the integrated circuit design. Afurther step 206, 706 includes generating a parameterized netlistcorresponding to each of the sample points and running a technologycomputer aided design simulation for each of the parameterized netlists,using the single finite element mesh for each of the parameterizednetlists, until convergence is achieved, to obtain, for each of theparameterized netlists, at least one metric indicative of theperformance of the integrated circuit design. A further step 210, 716includes developing a predicted design yield for the integrated circuitdesign, based on the at least one metric determined for each of theparameterized netlists.

In at least some instances, the identification of the plurality ofsampling points includes selecting such sampling points via randomand/or quasi-random statistical sampling. Such sampling can include, forexample, standard sampling such as Monte Carlo sampling, and/or fastsampling such as variance reduction. The skilled artisan will befamiliar with random statistical sampling, quasi-random statisticalsampling, standard sampling such as Monte Carlo sampling, and fastsampling such as variance reduction, and, given the teachings herein,will be able to implement one or more embodiments of the invention usingone or more of the mentioned sampling techniques.

With reference to FIG. 8, by way of review with regard to the aspectwherein only the single mesh is used, meshes require solving partialdifferential equations for the finite element method. However, in one ormore embodiments, the TCAD physical model does not need additionalrepeated mesh generation processes. FIG. 8 shows at 802, 804 some of thepartial differential equations for given meshes used in TCAD. Note thatμ_(n), μ_(p), D_(n), and D_(p) are exemplary physical parameters used inTCAD, and represent, respectively, electron mobility, hole mobility,electron diffusivity, and hole diffusivity.

In the case of importance sampling, the identifying step 704 is a firstidentifying step in which the sample points comprise first sample pointsand are uniformly distributed; and the generating step and running stepin step 706 are first running and first generating steps. Additionalsteps include step 708, computing a center of gravity of fails noted inthe first running step; step 710, namely, based on the center of gravityof fails, identifying a second set of sample points via importancesampling; and step 712. Step 712 includes generating a second set ofparameterized netlists corresponding to each of the second samplepoints; and running a technology computer aided design simulation foreach of the second parameterized netlists, using the single finiteelement mesh for each of the second parameterized netlists, untilconvergence is achieved (in step 714), to obtain, for each of the secondparameterized netlists, at least one metric indicative of theperformance of the integrated circuit design. The predicted design yieldin step 716 is further based on the at least one metric determined foreach of the second parameterized netlists (i.e., in the case of a singleloop in FIG. 2, the predicted design yield is directly based on themetrics from step 206 whereas in the importance based sampling approachof FIG. 7, the predicted design yield is directly based on the metricsfrom step 712 and indirectly based on the metrics from step 706, whichwere used as an intermediate result to obtain the final yield estimate716.

In one or more embodiments of the method, the first and second sets ofnetlists are actual values of device dimensions. The at least oneparameter can be, by way of example and not limitation, one or more of:gate length, channel width, random dopant fluctuation, line edgeroughness, oxide thickness, parasitic capacitance, parasitic resistance,distributive capacitance, distributive resistance, supply voltage,cell-supply voltage, chip temperature, negative bias temperatureinstability, positive bias temperature instability, and hot carriersinjection.

In any of steps 206, 706, 712, an additional step can include executingindividual simulations corresponding to given ones of the first andsecond parameterized netlists on at least two separate processors undercontrol of at least a third processor; for example, as per FIG. 3.

Exemplary System and Article of Manufacture Details

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

One or more embodiments of the invention, or elements thereof, can beimplemented in the form of an apparatus including a memory and at leastone processor that is coupled to the memory and operative to performexemplary method steps.

One or more embodiments can make use of software running on a generalpurpose computer or workstation. With reference to FIG. 6, such animplementation might employ, for example, a processor 602, a memory 604,and an input/output interface formed, for example, by a display 606 anda keyboard 608. The term “processor” as used herein is intended toinclude any processing device, such as, for example, one that includes aCPU (central processing unit) and/or other forms of processingcircuitry. Further, the term “processor” may refer to more than oneindividual processor. The term “memory” is intended to include memoryassociated with a processor or CPU, such as, for example, RAM (randomaccess memory), ROM (read only memory), a fixed memory device (forexample, hard drive), a removable memory device (for example, diskette),a flash memory and the like. In addition, the phrase “input/outputinterface” as used herein, is intended to include, for example, one ormore mechanisms for inputting data to the processing unit (for example,mouse), and one or more mechanisms for providing results associated withthe processing unit (for example, printer). The processor 602, memory604, and input/output interface such as display 606 and keyboard 608 canbe interconnected, for example, via bus 610 as part of a data processingunit 612. Suitable interconnections, for example via bus 610, can alsobe provided to a network interface 614, such as a network card, whichcan be provided to interface with a computer network, and to a mediainterface 616, such as a diskette or CD-ROM drive, which can be providedto interface with media 618.

Accordingly, computer software including instructions or code forperforming the methodologies of the invention, as described herein, maybe stored in one or more of the associated memory devices (for example,ROM, fixed or removable memory) and, when ready to be utilized, loadedin part or in whole (for example, into RAM) and implemented by a CPU.Such software could include, but is not limited to, firmware, residentsoftware, microcode, and the like.

A data processing system suitable for storing and/or executing programcode will include at least one processor 602 coupled directly orindirectly to memory elements 604 through a system bus 610. The memoryelements can include local memory employed during actual implementationof the program code, bulk storage, and cache memories which providetemporary storage of at least some program code in order to reduce thenumber of times code must be retrieved from bulk storage duringimplementation.

Input/output or I/O devices (including but not limited to keyboards 608,displays 606, pointing devices, and the like) can be coupled to thesystem either directly (such as via bus 610) or through intervening I/Ocontrollers (omitted for clarity).

Network adapters such as network interface 614 may also be coupled tothe system to enable the data processing system to become coupled toother data processing systems or remote printers or storage devicesthrough intervening private or public networks. Modems, cable modem andEthernet cards are just a few of the currently available types ofnetwork adapters.

As used herein, including the claims, a “server” includes a physicaldata processing system (for example, system 612 as shown in FIG. 6)running a server program. It will be understood that such a physicalserver may or may not include a display and keyboard.

As noted, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon. Anycombination of one or more computer readable medium(s) may be utilized.The computer readable medium may be a computer readable signal medium ora computer readable storage medium. A computer readable storage mediummay be, for example, but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, or semiconductor system, apparatus,or device, or any suitable combination of the foregoing. Media block 618is a non-limiting example. More specific examples (a non-exhaustivelist) of the computer readable storage medium would include thefollowing: an electrical connection having one or more wires, a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), an optical fiber, a portable compact disc read-onlymemory (CD-ROM), an optical storage device, a magnetic storage device,or any suitable combination of the foregoing. In the context of thisdocument, a computer readable storage medium may be any tangible mediumthat can contain, or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

It should be noted that any of the methods described herein can includean additional step of providing a system comprising distinct softwaremodules embodied on a computer readable storage medium; the modules caninclude, for example, any or all of the elements depicted in the blockdiagrams of FIGS. 1 and 3 or modules configured to carry out any one,some, or all of the steps illustrated in the flow charts of FIGS. 2 and7; by way of example and not limitation, a TCAD module, a TCAD-MIS (orother statistical) interface module, and an MIS (or other statistical)module. Furthermore, where parallelization is employed, the TCAD modulecan include a control module running as in block 304 and individualserver modules running as in block 302. The method steps can then becarried out using the distinct software module(s) and/or sub-modules ofthe system, as described above, executing on one or more hardwareprocessors 602. For example, steps 202 and 702 can be carried out by theTCAD module executing on at least one hardware processor, steps 204,704, and 710 can be carried out by the statistical sampling moduleexecuting on the at least one hardware processor; netlist generatingsteps can be carried out by the TCAD-statistical interface moduleexecuting on the at least one hardware processor; steps 206, 706, and712 can be carried out by the TCAD module executing on the at least onehardware processor, and step 708 can be carried out by the statisticalsampling module executing on the at least one hardware processor.Furthermore, where parallelization as in FIG. 3 is employed, steps 206,706, 712 can be carried out by at least two of the individual servermodules under control of the control module executing on the at leastone hardware processor. Further, a computer program product can includea computer-readable storage medium with code adapted to be implementedto carry out one or more method steps described herein, including theprovision of the system with the distinct software modules. Yet further,the data manipulated in the blocks and steps in FIGS. 1-3 and 7 isrepresentative of physical devices, circuits, and packages.

In any case, it should be understood that the components illustratedherein may be implemented in various forms of hardware, software, orcombinations thereof; for example, application specific integratedcircuit(s) (ASICS), functional circuitry, one or more appropriatelyprogrammed general purpose digital computers with associated memory, andthe like. Given the teachings of the invention provided herein, one ofordinary skill in the related art will be able to contemplate otherimplementations of the components of the invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method comprising: generating a single finiteelement mesh for predicting performance of an integrated circuit design;identifying a plurality of sample points for conducting a variabilitystudy on at least one parameter associated with said integrated circuitdesign, said sample points being selected to predict performance of saidintegrated circuit design when subject to variations in said at leastone parameter due to variations in manufacturing processes to be used tomanufacture said integrated circuit design; generating a parameterizednetlist corresponding to each of said sample points; running atechnology computer aided design simulation for each of saidparameterized netlists, using said single finite element mesh for eachof said parameterized netlists, wherein said single finite element meshremains unchanged for different parameters of each of said parameterizednetlists, until convergence is achieved, to obtain, for each of saidparameterized netlists, at least one metric indicative of saidperformance of said integrated circuit design; and developing apredicted design yield for said integrated circuit design, based on saidat least one metric determined for each of said parameterized netlists.2. The method of claim 1, wherein said identification of said pluralityof sampling points comprises selecting said sampling points via at leastone of random and quasi-random statistical sampling.
 3. The method ofclaim 2, wherein: said identifying step comprises a first identifyingstep in which said sample points comprise first sample points and areuniformly distributed; said generating step comprises a first generatingstep; and said running step comprises a first running step; furthercomprising the additional steps of: computing a center of gravity offails noted in aid first running step; based on said center of gravityof fails, identifying a second set of sample points via importancesampling; generating a second set of parameterized netlistscorresponding to each of said second sample points; and running atechnology computer aided design simulation for each of said secondparameterized netlists, using said single finite element mesh for eachof said second parameterized netlists, until convergence is achieved, toobtain, for each of said second parameterized netlists, at least onemetric indicative of said performance of said integrated circuit design;wherein said predicted design yield is further based on said at leastone metric determined for each of said second parameterized netlists. 4.The method of claim 3, wherein said first and second sets of netlistscomprise actual values of device dimensions.
 5. The method of claim 4,wherein said at least one parameter comprises at least one of: gatelength; channel width; random dopant fluctuation; line edge roughness;oxide thickness; parasitic capacitance; parasitic resistance;distributive capacitance; distributive resistance; supply voltage;cell-supply voltage; chip temperature; negative bias temperatureinstability; positive bias temperature instability; and hot carriersinjection.
 6. The method of claim 4, further comprising the additionalstep of, within at least one of said first and second running steps,executing individual simulations corresponding to given ones of saidfirst and second parameterized netlists on at least two separateprocessors under control of at least a third processor.
 7. The method ofclaim 6, further comprising providing a system, wherein the systemcomprises distinct software modules, each of the distinct softwaremodules being embodied on a computer-readable storage medium, andwherein the distinct software modules comprise a TCAD module, astatistical sampling module, and a TCAD-statistical interface module,said TCAD module in turn comprising a control module and a plurality ofindividual server modules; wherein: said mesh generating step is carriedout by said TCAD module executing on at least one hardware processor;said identifying steps are carried out by said statistical samplingmodule executing on said at least one hardware processor; said netlistgenerating steps are carried out by said TCAD-statistical interfacemodule executing on said at least one hardware processor; said runningsteps are carried out by at least two of said individual server modulesunder control of said control module executing on said at least onehardware processor; and said center of gravity computing step is carriedout by said statistical sampling module executing on said at least onehardware processor.
 8. The method of claim 2, further comprisingproviding a system, wherein the system comprises distinct softwaremodules, each of the distinct software modules being embodied on acomputer-readable storage medium, and wherein the distinct softwaremodules comprise a TCAD module, a statistical sampling module, and aTCAD-statistical interface module; wherein: said mesh generating step iscarried out by said TCAD module executing on at least one hardwareprocessor; said identifying step is carried out by said statisticalsampling module executing on said at least one hardware processor; saidnetlist generating step is carried out by said TCAD-statisticalinterface module executing on said at least one hardware processor; andsaid running step is carried out by said TCAD module executing on saidat least one hardware processor.